Conventional crash detection arrangements typically comprise a crash sensor and a control unit. The crash sensor is usually an accelerometer which is connected to a processor within the control unit to provide a signal to the processor which is indicative of the acceleration applied to the vehicle, for instance by forces arising during a crash situation. The control unit is usually provided with a first comparator which compares the signal from the accelerometer with a predetermined acceleration value, which is set at a level such that values of acceleration higher than the predetermined value would indicate that the vehicle is involved in a crash situation. The processor is configured to process the signal from the accelerometer when the first comparator indicates that the signal from the accelerometer is in excess of the predetermined acceleration value. The processor processes the signal for a predetermined length of time (as explained in more detail below) following a determination that the acceleration has first risen above the predetermined acceleration value, and this processing generally determines an amount by which the velocity of the vehicle changes during the predetermined length of time.
A second comparator compares the result of the processing of the signal with a predetermined threshold. If the second comparator finds the result of the signal processing to be in excess of the predetermined threshold (i.e. the velocity has changed by more than a pre-set amount), the second comparator generates a trigger signal which is indicative of the occurrence of a crash situation which is severe enough to warrant activation of a safety device, such as an air-bag. The trigger signal is then transmitted to a safety device to actuate the safety device to protect an occupant of the vehicle.
Referring now to FIG. 1 of the accompanying drawings, the variation in the acceleration a of a vehicle is plotted against time during a crash, with a first curve a1 which corresponds to a high speed crash (e.g., a crash at 37 mph) and a second curve a2 which corresponds to a relatively low speed crash (e.g., a crash at 9 mph).
If a conventional crash detection arrangement, such as the arrangement described above, is installed in a vehicle which is involved in either the high speed crash or the low speed crash, the processor will begin to process the acceleration signal a1 or a2 when first comparator indicates that the acceleration signal a1 or a2 is in excess of a predetermined acceleration value a0. The times at which the processor starts to process the acceleration signal a1 or a2, are indicated respectively at times t01 and t02 on FIG. 1.
The processor processes the signal a1 or a2 by integrating the signal a1 or a2 over a set length of time to determine the change in velocity Δv of the vehicle, according to the following equation:Δv=(∫(a−a0)dt)
The resultant value indicative of the change in velocity Δv is then compared, by the second comparator, with a predetermined threshold ΔvT.
If the second comparator detects the change in velocity Δv to be in excess of the predetermined threshold ΔvT, (i.e. Δv>ΔvT) the second comparator generates a trigger signal which is transmitted to the safety device to actuate the safety device to protect an occupant of the vehicle.
Referring now to FIG. 2, the change in velocity Δv is plotted against time, with a first curve Δv1 corresponding to the integral of the first curve a1 of FIG. 1, (i.e. the high speed crash) and a second curve Δv2 corresponding to the integral of the second curve a2 of FIG. 1 (i.e. the low speed crash). It can be seen, from FIG. 2, that the curves Δv1 and Δv2 each start at the respective times t01 and t02, which each correspond to the times at which the acceleration a1 or a2 first exceeds the predetermined acceleration value a0, and hence the time at which the processor starts processing the signal a1 or a2.
A determination must be made within an appropriate period of time (e.g. 30 ms) following the time at which the acceleration rises above a0 as to whether the crash situation requires the actuation of a safety device. If the actuation of the safety device is not triggered within an appropriately short period of time, the benefit of the safety device may be lost and the actuation thereof may be positively harmful to a vehicle occupant.
As can be seen from FIG. 2, the changes in velocity Δv1 and Δv2 exceed the predetermined threshold ΔvT for respectively the high speed crash and the low speed crash at respective trigger times tT1 and tT2, which correspond to approximately 30 ms after t01 and t02 respectively.
The actuation of the safety device is desirable in the case of the high speed crash, represented by the first curves a1 and Δv1, as the forces (proportional to a1) arising from such a high speed crash will become large, at the end of the crash event meaning that it is likely that an occupant will need protection. However, it may not be desirable to trigger the safety device as a result of the low speed crash represented by the second curves a2 and Δv2, as it is unlikely that an occupant will need the protection provided by the safety device since the forces arising from the low speed crash are likely to be minimal.
Unfortunately, as can be seen from FIGS. 1 and 2, the acceleration in a 30 ms time period following the moment at which the acceleration rises above the threshold is largely dependent on the stiffness of parts of the vehicle, and is not heavily dependent upon the severity of the impact. In a severe impact, such as a high speed crash, the acceleration will continue to rise after the 30 ms interval has passed (following a short “plateau” phase) but, as discussed above, it is not desirable to wait longer than the 30 ms interval this before a decision must be taken as to whether to actuate the safety device.
In the case of a high speed crash, therefore, it is important to actuate the safety device very soon after the crash has occurred, so that the safety device may be fully deployed to protect an occupant of the vehicle from forces arising from the crash. In order to provide early actuation of the safety device, the predetermined threshold ΔvT can be set at a relatively low level so that the time taken for the change in velocity Δv to rise to the low predetermined threshold ΔvT is relatively short. However, if a low value of the predetermined threshold ΔvT is chosen, the change in velocity Δv2 in the case of a low speed crash will also rise to the level of the predetermined threshold ΔvT during the 30 ms processing period. Thus, selecting a low value for the predetermined threshold ΔvT can result in the safety device being unnecessarily actuated in the event of a low speed crash.
One way to prevent the safety device from being actuated in the event of a low speed crash would be to raise the predetermined threshold ΔvT to a level which the change in velocity Δv2 in a low speed crash will not reach. In this case, the safety device would only be actuated by the relatively large change in velocity Δv1, arising from a high speed crash, which reaches the higher predetermined threshold ΔvT. However, the raising of the predetermined threshold ΔvT means that the length of time until which the change in velocity Δv takes to reach the threshold is increased, thus increasing the length of time after which the crash occurs when the safety device is actuated.
Therefore, the need arises for a crash detection arrangement which can actuate a safety device swiftly to protect an occupant of a vehicle during a high speed crash, but which will not actuate the safety device unnecessarily in response to a relatively low speed crash.
The present invention seeks to provide an improved arrangement for determining the severity of a crash at an early stage.
According to one aspect of the present invention, there is provided a crash detection arrangement, to be installed in a motor vehicle, for detecting a crash and providing a control signal for controlling a safety device in the event that a crash is detected, the arrangement comprising an accelerometer and a control unit, the accelerometer being arranged to supply a signal to the control unit which is indicative of the acceleration of the vehicle, characterised by the control unit being adapted to: calculate a classification parameter based on the value of the signal from the accelerometer during a classification time period, which includes an interval of time before an initiation criterion was fulfilled; modify a crash evaluation algorithm in dependence upon the classification parameter; and perform the crash evaluation algorithm upon fulfillment of the initiation criterion to produce the control signal.
Advantageously, the control unit is adapted to compare the signal from the accelerometer with a predetermined acceleration value, and the initiation criterion is fulfilled when the signal from the accelerometer first exceeds the predetermined acceleration value.
Preferably, the crash evaluation algorithm comprises processing of the signal from the accelerometer for an evaluation time period which follows the time at which the initiation criterion is fulfilled.
Conveniently, the control signal comprises an actuation signal, an evaluation parameter is calculated by the crash evaluation algorithm, the step of modifying the crash evaluation algorithm comprises the steps of setting a threshold value in dependence upon the value of the classification parameter, and the crash evaluation algorithm comprises comparing the evaluation parameter with the threshold value to provide an actuation signal in dependence upon the result of the comparison.
Advantageously, the crash evaluation algorithm comprises integration of the sensed value of acceleration, and an actuation signal is provided if the result of the integration is greater than the threshold value.
Conveniently, the control unit is configured to set the threshold value to be equal to a low threshold value when the classification parameter indicates that the rise in acceleration before fulfillment of the initiation criterion is relatively fast, and to be equal to a high threshold value when the classification parameter indicates that the rise in acceleration before fulfillment of the initiation criterion is relatively slow.
Advantageously, the classification parameter is based at least partly on an integration of the sensed value of acceleration during the classification time period, or on an average of the sensed value of acceleration during the classification time period.
Preferably, the control unit is configured to set the threshold value to be equal to the high threshold value when the classification parameter is relatively high and to be equal to the low threshold value when the classification parameter is relatively low.
Alternatively, the classification parameter is based partly on an average of a derivative of the sensed value of acceleration during the classification time period.
Conveniently, the control unit is configured to set the threshold value to be equal to the high threshold value when the classification parameter is relatively low and to be equal to the low threshold value when the classification parameter is relatively high.
Advantageously, the determination as to whether the classification parameter is relatively high or relatively low is made by comparing the classification parameter with a predetermined constant.
Preferably, the threshold value is proportional to the classification parameter.
Conveniently, the classification parameter provides an indication of the rapidity of the rise in acceleration before fulfillment of the initiation criterion.
Advantageously, the control unit is configured to set the threshold value according to a formula which is dependent upon the classification parameter.
Preferably, the control signal comprises a variable output value.
Conveniently, the control unit repeatedly re-calculates the classification parameter.
Advantageously, the classification parameter is re-calculated at regular intervals.
Preferably, the classification parameter is calculated in response to the fulfillment of the initiation criterion.
Conveniently, the arrangement comprises a memory which is configured to store sensed values of acceleration.
Advantageously, the memory is configured to store, at a given moment, values of acceleration that were sensed during a predetermined length of time preceding the given moment.
Preferably, upon fulfillment of the initiation criterion, the predetermined length of time corresponds to the classification time period.
Conveniently, the classification parameter is calculated using values of acceleration stored in the memory.
Advantageously, the classification time period falls entirely before the fulfillment of the initiation criterion.
Preferably, the classification time period has a length of approximately 8 ms.
Another aspect of the present invention provides a crash detection method for detecting whether a vehicle is involved in a crash and providing a control signal for controlling of a safety device in the event that a crash is detected, the method comprising the step of: providing an accelerometer which supplies a signal which is indicative of the acceleration of the vehicle; and being characterised by the steps of calculating a classification parameter based on the value of the signal from the accelerometer during a classification time period, which includes an interval of time before an initiation criterion was fulfilled; modifying a crash evaluation algorithm in dependence upon the classification parameter; and upon fulfillment of the initiation criterion, performing the crash evaluation algorithm to produce the control signal.